keygens

Typically software requests a serial number only during installation. The installation software applies an algebraic expression to the key entered to validate the key. As an example, the algorithm may define that a key entered must contain 5 numbers that have a checksum of 25, and that the key must also contain 3–5 letters that, when mapped into their numerical equivalents, have a sum of 42.

In order to write a keygen, the author typically uses a dissasemblerto look at the raw assembly code of the program they are writing the keygen for, checking either the software itself or the installer. Once they have access to the program's code, they can locate the subroutine(s) responsible for verifying that the key entered is valid. Using this knowledge, they can reverse the algorithm used to generate valid keys, which is then incorporated into the keygen. -wikipedia.org

to prevent crackers from making keygens, software companies included online registration (like adobe). some can detect a pirated software copy by checking the serial number used to register the product. if the serial number was used second time, it is automatically detected as a pirated copy (like kaspersky). on the other hand, norton focuses on the vulnerability of its code. they focused on protecting the code itself, making it hard to locate or to view to prevent crackers from making keygens.